With the growth of the wireless communications industry, wireless communications protocols become more sophisticated and demanding in their requirements for complex modulation schemes and narrow channel bandwidths. The ultimate goal is to encode as much digital information as possible in a given channel. One such modulation scheme for encoding digital information is polar modulation. Polar modulated RF transmitters utilize both amplitude modulation (AM) and phase modulation (PM) to maximize the amount of information that can be encoded with minimum bandwidth. By using multiple combinations of phase and amplitude modulation, multiple digital bits of information can be represented. Large signal amplitude modulation allows several distinct levels of modulation with adequate noise margins for reliable encoding of digital data. One such polar modulation technique is called 8 Phase Shift Keying (8-PSK).
In a polar modulated system, large signal amplitude modulation can affect proper operation of phase modulated signals. In addition, large signal amplitude modulation may have non-linearities in the relationship between an amplitude control signal and output power. Actual output power must stay within specified tolerances. One measure of merit in a polar modulated transmitter is called Error Vector Magnitude (EVM), which represents the error between actual polar modulated output signals and ideal polar modulated output signals. For reliable operation in some polar modulated systems, certain EVM values must be maintained within maximum specifications. In addition, the bandwidth of transmitted polar modulated RF signals must be contained within a single channel, and not interfere with adjacent channels. Output Radio Frequency Spectrum (ORFS) is a measure of adjacent channel interference, which must be maintained within maximum specifications. Thus, there is a need for a polar modulated RF transmitter that conforms to RF output power tolerances, meets EVM maximum specifications, and meets ORFS requirements.